The present invention relates generally to fluid assisted injection molding processes, and more particularly to such a process utilizing an overfill reservoir selectively connectable to a fluent plastic supply line.
There are a wide variety of gas or fluid assisted injection molding apparatuses and processes available in the art. Injection molding generally comprises injecting a molten plastic under pressure (usually by a screw feed injector) into a closed two piece cavity. When the part cools, the mold pieces are separated and the part removed. There are various references to specific pressure profiles to best implement the molding process, and a plethora of plastic injection molding machines commercially available.
Gas or fluid assisted injection molding generally involves injecting gas into the fluid plastic material either during or after plastic injection to create a hollow within the part. This reduces the weight of the part and the cost of material used. More importantly, pressurizing the interior of the part forces the fluid plastic against the mold surface as it cools. When plastics cools, it shrinks, and tends and pull away from the mold surface, leaving unsightly sink marks. The cooling of the plastic within the mold also reduces the pressure of the plastic within the mold. There are a variety of gas or fluid assist controllers and equipment commercially available.
There is another variation of the injection process known generally as overflow, overspill, spillovers or similar names. This process generally involves injecting more plastic material into the mold cavity than the cavity will hold, and allowing material to flow into reservoirs at the remote ends of the plastic flow path to receive the excess. If the reservoir locations are chosen properly, the plastic must fill every bit of the mold cavity before the reservoirs are filled, thus ensuring complete mold fill out. Again, molding equipment utilizing the overflow concept is commercially available.
Some combinations of overflow and fluid injection have been attempted, generally to speed the fill out process or to intentionally dispel fluid plastic from the part interior to create a hollow part. These processes have generally proven unreliable (poor repeatability). The typical combination process injects gas at or near the plastic inlet, pushing the plastic toward the overspill at the far end(s) of the mold cavity. This results in a flow of the cooling resin toward a small gate located at the opposite end of the cavity. When the resin cools, it is much less viscous and tends to resist flowing through the overspill gate. The plastic""s resistance to shear also increases with the decrease in temperature, adding further resistance to travel through the overspill gate, and causing the resin flow to stall at the overspill entrance. This xe2x80x9cblockage,xe2x80x9d or area of greater resistance to flow, can lead to or cause a number of problems or undesirable conditions. For example, this situation often prompts operators to utilize unnecessarily high gas injection pressures to move the resin through the overspill gate. Further, this undesired resistance may localize high gloss areas over the channel.
Typically, when confronted by the resistance of the cooling resin at the over-spill gate, the gas will in effect migrate to xe2x80x9cthin wallxe2x80x9d sections of the plastic part causing quality/function problems. This is like blowing up a balloon with thin spots, the thicker areas will not stretch, causing the thin section to overstretch. As a result, parts are characterized by an increase in the resin wall thickness as the gas moves from the hotter gate area at the point of gas injection (more pliable resin is moved along by the gas) to the relatively cooler area at the end of the gas channel/entrance of the overspill (less pliable resin stays in place and is less affected by the gas). Further, if the amount of plastic flowing into the overspill is reduced, the amount of space the gas will occupy at a given pressure is similarly reduced, thus yielding a part heavier than desired. Further still, the use of gas injection at/near the point of plastic injection creates a need to have greater or even excessive gas injection delay times to insure that the hotter resin around the gate/pin is cooled sufficiently that the molten resin will not be blown off the gas pin. Similarly, longer gas injection delay times would also be necessary to ensure that the hotter resin around the gate/pin is cooled sufficiently so that the molten resin will not xe2x80x9cfoam upxe2x80x9d (become mixed with resin). The higher the gas pressure to be used, the longer the injection delay required to avoid these problems.
U.S. Pat. No. 5,204,051 to Jaroschek is entitled xe2x80x9cProcess For The Injection Molding Of Fluid-Filled Plastic Bodies.xe2x80x9d In Jaroschek, a, flowable plastic melt is first injected into a mold cavity. After cooling of the plastic melt along the mold cavity walls, a fluid is injected in a manner such that the still-melted center of the resulting plastic body is expelled into a side cavity. Jaroschek states that the process can be undertaken in such a way that fluent plastic is forced back toward the plastic supply by the incoming fluid. Thus, the molten plastic supply could serve as the side cavity for receipt of the expelled plastic; however, it is first necessary to lift the sprue away from its seat to allow the plastic to pass, leaving a quantity of plastic between the sprue body and its seat.
In one aspect, an injection molding apparatus is provided. The injection molding apparatus includes a cavity for forming a hollow molded plastic part, a source of fluent plastic fluidly connectable to the cavity, and a runner for supplying fluent plastic from the source to the cavity. At least one fluid injection pin is provided and is mounted to the mold body and connectable to a fluid source. A reservoir is also provided and is positioned remote from the cavity, the reservoir is selectively connectable to the runner via a sub-runner. Finally, a valve is positioned adjacent a mouth of the sub-runner. The valve is operable between a first state at which the reservoir is fluidly connected to the runner and a second state at which the reservoir is blocked from fluid communication with the runner.
In another aspect, a process for injection molding of fluid filled plastic bodies is provided. The process includes the steps of providing an injection molding apparatus having a mold body that defines a mold cavity, and a source of flowable plastic material fluidly connectable to the mold cavity with a supply passage. At least one reservoir is also provided and is fluidly connectable to the supply passage with a control valve. At least one fluid injection pin is also provided and is connectable to a fluid source. The process further includes the steps of injecting a quantity of flowable plastic into an interior of the mold cavity through the supply passage, and cooling part of the injected plastic along the walls of the mold cavity, providing an interior of flowable plastic melt. In addition, the process includes the step of selectively expelling at least a portion of the interior of flowable plastic melt into the supply passage, and selectively expelling at least a portion of fluent plastic from the supply passage into the reservoir.
In yet another aspect, a method of forming a hollow injection molded plastic part is provided. The method includes the steps of providing a mold body having a mold cavity, connecting a source of fluent plastic to the mold cavity with a runner passage, and mounting at least one fluid injection pin to the mold body, and connecting the pin to a fluid source. The method further includes the steps of injecting a quantity of fluent plastic via the runner into the mold cavity, and injecting a quantity of fluid into the mold cavity, thereby expelling a portion of the quantity of fluent plastic to the runner, leaving a hollow plastic body around the periphery of the mold cavity. The method finally includes the step of selectively connecting the runner to a reservoir and expelling a quantity of fluent plastic to the reservoir.